Such cables are used in particular when drilling for oil, and in service they are subjected to very severe stresses due both to the pressure exerted by the layers of rock through which the cable passes, and also to the high temperatures which are due in part to the natural increase of temperature with depth, and in part to the heat generated by the drilling work.
There is now a requirement for cables that are capable of withstanding service temperatures higher than 200.degree. C., and even higher than 220.degree. C., and pressures of several hundred atmospheres.
Single conductor insulants capable of withstanding such service conditions are already known, for example: polymethylpentene as claimed in the present Assignee's published French patent application No. 2,472,823; ethylene or propylene polyfluoride; and the copolymer of ethylene and tetrafluroethylene.
In contrast, fillers that perform satisfactorily under such circumstances are not known. In this context, satisfactory performance includes uniform distribution of potential inside the cable, adequate stability at the high temperatures encountered, and satisfactory manufacturing properties.
Furthermore, such fillers are called on to be elastic and to oppose deformation of the insulants under the effect of mechanical and thermal stress.
In particular, a composite of polyethylene tetrasulphide charged with carbon black cannot withstand temperatures above 150.degree. C. The carbon black is used to provide electrical shielding, uniform distribution of potential around the different conductors, and to evacuate static charges that might otherwise disturb measurements.
Preferred embodiments of the present invention thus provide an electromechanical cable capable of withstanding at least 200.degree. C., and preferably 220.degree. C., at pressures of several hundred atmospheres.